The present invention relates to energy spectroscopy systems and more particularly to correcting the output of solid state radiation detectors.
A significant context contemplated for the present invention is energy spectroscopy. In an nominal system, a solid state detector is connected to a preamplifier providing output pulses indicative of radiation detected by the detector. The preamplifier output is provided to an amplifier, which may perform prior art pulse shaping functions and provide an output to a well-known multichannel analyzer. The multichannel analyzer performs many functions required for most nuclear spectroscopy measurements. Most typically, a multichannel analyzer is used in the pulse height analysis mode for accumulating a spectrum, or histogram, of the frequency distribution of the heights from a sequence of input pulses. Nuclear and x-ray energy spectroscopy and time spectroscopy comprise the majority of applications for which the pulse height analysis mode is used. The desired spectrum is accumulated by measuring the amplitude of each input event, converting it to a number that is proportional to the pulse height and storing information in a memory comprised of individual channels. The number of counts in each channel is equal to the total number of pulses processed whose amplitudes are in a range corresponding to the channel number. The range width customarily corresponds to the resolution of analog to digital conversion circuitry in the multichannel analyzer. As the gamma ray energy is characterized in terms of the voltage from the spectroscopy amplifier. The spectrum comprises the number of pulses versus the channel number obtained in response to digitizing the pulses. In one form, each value resolvable by analog to digital conversion circuitry may comprise one channel.
One factor affecting resolution of such a spectroscopy system is the inherent capability of a system to provide at the spectroscopy preamplifier terminal a pulse accurately representing the event that caused the detector to provide the output pulse. As recent developments in the art have enabled further refinements in resolution of such spectroscopy systems to provide for correction factors to more accurately represent each event to the multichannel analyzer. The correction factor compensation can be incorporated in the spectroscopy amplifier which couples the preamplifier to the multichannel analyzer. Advances in the art have enabled correction for physical phenomenon occurring in the detector itself.
One prior art effect is known as ballistic deficit. The phenomenon and a correction therefor is explained, for example, in F. S. Goulding and D. A. Landis, Ballistic Deficit Correction in Semiconductor Detector Spectrometers, publication LBL 22195, Lawrence Berkeley Laboratory, University of California, Berkeley, Calif. 94720, October 1987. The phenomenon is further described in Billy Loo, F. S. Goulding and Dexi Gao, Ballistic Deficits in Pulse Shaping Amplifiers, publication LBL-23356, September 1987, Lawrence Berkeley Laboratory.
A ballistic signal is an output whose amplitude is proportional to the total charge that appeared on the collection electrode of the detector irrespective of the time profile of the charge arrival, such as in a ballistic galvanometer. While the measurement of a signal produced by a semiconductor detector should ideally be a ballistic measurement, this is difficult to achieve in the case of a spectrometer where pulse shaping circuits used in spectrometers are designed to produce output pulses whose total duration is usually limited to a few microseconds. The peak of the output pulse, which occurs at a time less than half the pulse width is used as a measure of the input charge signal from the detector. For large, coaxial germanium gamma ray detectors and thick silicon charged particle detectors, charge collection times fluctuate between events depending on the location of interactions. More qualitatively stated, the same event, i.e. absorption of a gamma ray in the detector may produce a different pulse amplitude depending upon the point of interaction in the detector.
"Ballistic deficit effects" result in fluctuations in the detector charge signal rise times, which result in fluctuations in the amplitudes of the spectroscopy amplifier pulses. While system resolution may be limited by electronic noise at low energies, more subtle effects such as ballistic deficit may become dominant at higher energies.
These same investigators also recognize that in semiconductor detectors, fluctuations in the peaking time of the input signals are often results of distribution of charge origins within a detector volume, field inhomogeneities or charge trapping. From the point of gamma ray interaction, majority and minority carriers must migrate to their respective electrodes. The carrier which travels the longer distance controls the preamplifier, or charge signal, rise time. The further this charge must migrate from where the gamma ray is absorbed, to the electrode, the greater will be the charge signal rise time sensed at the preamplifier output. This fluctuation in charge signal rise times affects proportionately the peak amplitude of the spectroscopy amplifier output. This effect on the spectroscopy amplifier output is characterized as the ballistic deficit. One circuit is proposed to compensate for relative ballistic deficit by correcting the pulse amplitude provided to the multichannel analyzer. The degree of correction necessary is characterized by the peak signal amplitude deficit. The peak signal amplitude deficit is related to the peak amplitude for a zero rise time signal times the square of the pulse peak delay time divided by peaking time for output with zero rise time input signal. Pulse peak delay time is a function of the geometry of the detector related to the distance a charge must migrate to reach an electrode. This relationship is utilized to provide circuitry to provide for ballistic deficit correction. While charge trapping fluctuations are recognized, they are of a type which slow the charge but do not prevent its being recognized as a component represented in the preamplifier output. Additional corrections are not made therefor.
In the detector, radiation results in creation of a charge. Majority and minority charge carriers need to migrate from the point in the detector where they are generated to electrodes in order to be sensed. Charge traps are physical and electrical impediments which impede the migration of charge carriers from the initial location of the charge to the electrode. In use of the above-described circuit, it is assumed that charge trapping phenomenon is due to shallow traps whose time for capture and re-emission of charge carriers were shorter than the pulse processing time. Therefore, effects due to deep level traps which prevent charge from reaching the detector output during pulse processing are not accounted for. Consequently a further deficit phenomenon will affect the spectroscopy amplifier output. In accordance with the present invention, it is recognized that deep level trapping is encountered significantly more often in physical situations than shallow level trapping. Improvement is provided in resolution compared to correcting only for the sort of ballistic deficit effects described above is provided based on a deep trap model which does not assume that capture / emission times are substantially shorter than pulse processing time.